Conventionally, absolute position detectors used for applications such as control of a feed shaft of a machine tool have been known (for example, JP 2003-35566 A). Such an absolute position detector includes an absolute position detection sensor for detecting the absolute position of the shaft, and a high-resolution detection sensor for detecting the position at a resolution higher than that of the absolute position detection sensor. Based on output values obtained from these two types of sensors, a high-resolution absolute position is calculated.
It has been known that due to temperature drift and temperature characteristics of an interpolation circuit constituted with analog components, an angular error component that varies periodically is generated between the output value from the absolute position detection sensor and the output value from the high-resolution detection sensor. Conventionally, this angular error component was determined as a relative error, and when this relative error is excessively large, it was judged that an abnormality is generated.
FIG. 4 is a block diagram showing a conventional absolute position detector provided with such an abnormality detection function. FIG. 5 is a timing chart showing a case in which the abnormality detection process is performed.
A high-resolution position detection sensor 2 is a 4× resolver, and an absolute position detection sensor 3 is a 1× resolver. These sensors 2, 3 constitute a rotary position detector, and are mechanically coupled to a motor (not shown) via a shaft 1. In the absolute position detection sensor 3 which is a 1× resolver, every time the shaft 1 makes one full rotation, the phase of the detection signal is modulated 360°. On the other hand, in the high-resolution position detection sensor 2 which is a 4× resolver, every time the shaft 1 makes a ¼ rotation, the phase of the detection signal is modulated 360°. An interpolation circuit 4 transmits a magnetization signal to the absolute position detection sensor 3 in synchronization with a synchronization signal CL, to thereby perform interpolation with respect to the two-phase signal modulated in accordance with the position and to output an angle θ2 expressed in 8 bits. The synchronization signal CL is a signal obtained by dividing, into ¼, a synchronization signal CH supplied from a transmitter 7 using a ¼ frequency divider 8. Further, in synchronization with the synchronization signal CH from the transmitter 7, an interpolation circuit 5 similarly performs interpolation with respect to the signal from the high-resolution position detection sensor 2, so as to output an angle θ1 expressed in 8 bits.
In general, the high-resolution angle θ1 is aligned in places of digits with a counter circuit (not shown), then sampled at a high speed to be converted into a position detection value expressing a single-rotation absolute value, and then transmitted to a control device. The absolute-value detection angle θ2 is used for generating an initial value of the counter circuit at the time of turning ON the power. During normal operation, this angle θ2 is sampled at a low speed for use in abnormality detection.
An abnormality detection method is next explained. A relative error calculation circuit 6 calculates a relative error E from the angles θ1 and θ2 using the following formulas 1 and 2.X=(θ2·4−θ1)/28  (1)E=|X−INT(X)−0.5|  (2)
Here, INT( ) denotes a function that returns a maximum integer that does not exceed the numerical value inside the parentheses. For example, INT(1.9)=1 and INT(−1.9)=−2 hold true. An abnormality judgment unit 9 outputs an abnormality detection signal AF when the relative error E exceeds an abnormality judgment reference value. The abnormality judgment reference value is a preset value, and may be set to 0.3 or the like, for example.
An absolute position detector configured as described above is integrated into a motor and coupled to a ball screw of a feed shaft of a machining tool via a coupling. In recent years, machine tools have been further downsized, such that the spindle power line of a machine tool is often arranged in the vicinity of the feed shaft motor. This spindle power line is known to cause electromagnetic waves due to large inverter switching noise generated when performing regenerative operations at times of spindle deceleration.
Next it will be explained how this noise causes errors in a position detector. In FIG. 3, the dashed line shows the relative error E when there is no noise. As the above-described noise generally occurs intermittently at a high frequency, as shown in FIG. 3, when the noise overlaps the instances of sampling of the sensor signals of the position detector (i.e., the times shown by the vertical lines in the graph showing the relative error E), the error is increased to form a random waveform as shown by solid lines.
When electromagnetic waves interfere with the resolvers inside the absolute position detector integrated in the feed shaft motor, the position detection values become unstable and errors are generated. As a result, the value of the relative error E may exceed the abnormality judgment reference value, such that the abnormality detection signal AF may be output. Since an abnormality caused in this manner is not an intended object of abnormality detection, this should actually be ignored. However, in conventional devices, as it was impossible to distinguish whether the relative error E exceeded the reference value due to the switching noise or due to an actual abnormality, an alarm was output in either case.
Further, as shown in FIG. 3, the period for detecting the relative error E is generally longer than the switching period of the inverter, and may undesirably correspond to an integer multiple of the switching period. The switching noise could thus occur in synchronization with the instances of detection of the relative error, resulting in constant presence of the influence of the noise. In such cases, the probability of the relative error exceeding the abnormality judgment reference value becomes high.
Although it may be possible to overcome the above problems by designing and assembling the absolute position detector so that the accuracy of the respective resolvers are improved and the relative error becomes reduced, in that case, the problem of increase in the manufacturing cost of the absolute position detector becomes unavoidable.
In light of the above situation, the present invention is directed to providing an absolute position detector configured to detect abnormalities at a higher accuracy.